1. Field of the Invention
The present invention relates to a method of accurate shape working of the upset end portions of steel pipe used for oil country tubular goods.
2. Description of the Prior Art
Hitherto, oil country tubular goods have been assembled by joining many lengths of steel pipe. Accordingly, the ends of the pipe have been threaded or coupled so as to improve the assembling operation efficiency.
It is necessary to finish the ends of the pipe to a prescribed shape prior to working the ends of the pipe. As a conventional method of forming the ends of the pipe, internal upset forging is applied to the ends of the pipe so as to finish said ends to a prescribed shape. (Refer to Japanese published unexamined patent application Sho No. 215221/59 and 76638/59).
According to the conventional method of internal upset forging, the wall thickness t of the ends 1 of a steel pipe heated to a working temperature is increased to a wall thickness t.sub.0 by means of a die 2A and a mandrel 3A as shown in FIG. 1B, and said ends 1 are further thickened to wall thickness t.sub.0 +a by changing said mandrel 3A to mandrel 3B in accordance with the wall thickness to obtain the product.
Furthermore, Japanese published examined patent application Sho No. 46212/61 discloses a method of working wherein a steel pipe subjected to external upset forging is reduced by means of a reducer so as to form a thickened wall portion inwardly and further is subjected to external upset forging to form a thickened wall portion outwardly.
However, in the case of the former, when the internal upset forging is applied, it is difficult to control the shape of the internal upset portion having a taper 4 and the length of this portion (FIG. 1B) which is not restrained by the mandrel, and furthermore underfill and buckling are apt to occur at that portion. Underfill and buckling are causes of lower operational efficiency due to the necessity for repair of the damage by grinding to an allowable limit.
In the case where the degree of said underfill and buckling is large, it becomes necessary to apply reupset forging or to scrap the material, which leads to yield problems. Furthermore, when a bending or twisting load is applied to a steel pipe such as in the drilling of an oil well, stress is easily concentrated so that cracks are apt to occur on the portion having the underfill and buckling.
In the case of the latter, on the other hand, the metal of said pipe is displaced inwardly for the thickening after shaping by external upset forging, and accordingly said method is preferable to that of the former. However, the outside diameter of the upset portion is specified as conforming to the outside diameter of the parent pipe, because the reducer is employed, furthermore, a sufficient high temperature is required for the reducing, so reheating is also required. Also, in the case where an upset portion having a more complicated shape is required, there is also problem that an additional die or working is necessary.